The present invention relates to a relief valve. In particular, the present invention relates to a thermally activated relief valve for use with a compressed gas storage cylinder (or pressure vessel).
It is desirable for some compressed gas storage cylinders to have safety valves such as a thermal relief device (TRD). For example, cylinders which hold compressed natural gas are required by law to have such relief valves.
The advance of high pressure composite technology has made it feasible and economically attractive to expand the size and pressure capabilities of storage cylinders for compressed natural gas used in vehicular applications. These new design, light weight, composite cylinders are typically constructed using a thin wall metal or plastic liner which is overwrapped with multiple layers of fiberglass/epoxy resin or graphite fiber/epoxy resin. In either case, the majority of the cylinders' burst strength is provided by the fiberglass or graphite/resin overwrap. Because the overwrap material is more susceptible to damage in a fire than metal, this cylinder design is more vulnerable to fire than conventional all-metal storage cylinders.
To adequately protect these "space age" cylinders from wrap strength degradation in a fire, the cylinders must be protected/fitted with high flow, fast acting, thermal relief safety devices. It should be noted that older style thermal devices (either open throat or convoluted/mazed) do not function (activate) consistently in these applications because the relatively cold stored gas inside the "insulated" cylinder cools the flow blocking eutectic mix as the gas begins to exit. This causes the eutectic to "re-freeze" and block the gas exit path. In this scenario, the fire causes the outer wall temperature of the cylinder to continue to escalate, while the eutectic safety device goes through a succession of freeze/thaw (melt) cycles during which time (typically 8-12 minutes) very limited amounts of compressed gas are released. This situation leaves the cylinder very vulnerable to catastrophic failure because of the cylinders' reduced burst strength.
There is, therefore, a need for a relief valve which meets or exceeds all of the critical performance criteria necessary for these newer cylinder designs. Such a relief valve must satisfy the following conditions:
(1) Fast Action - Typical fire activation time of 2-3 minutes is needed to allow compressed gas to begin venting before the overwrapped cylinder's burst strength is significantly reduced.
(2) Very High Reliability - The design must have the following features to assure high reliability:
(a) The relatively cool exiting gas stream can not resolidify the eutectic to possibly cause a catastrophic failure due to delayed activation. PA1 (b) Cylinder gas pressure can not exert an "extruding" force on the eutectic when the eutectic becomes susceptible to "plastic flow" (creep) as the system temperature approaches/encroaches on the eutectic melt temperature (during routine operation).
(3) Relatively High Flow - The relief valve must open a relatively large exhaust port to achieve high flow (exhaust) rates.